About the solution -the only part that's really hacky is when you access _data[name] (because I assume this is a private data struct)
```js
relationshipIds = parent._data[name].mapBy('id');
```

That said, if it solves the problem and we have it under test (and it saves us from having to create a custom store) I say it might be the best tradeoff / option we have right now. (unless ember-data provides this data on the record in a "less hacky" way -in which case we simply refactor in the future)